Add expected failing test for illegal unicode chars

Will add fix later
ALso switch to pytest
install unittest2 to travis, try to import and fallback to unittest
Only install unittest2 for py 2.6
Add option to excape illegal chars

If PyToXml is called with escape_illegal_chars=True then the string
builder will catch a ValueError and replace with ?
Test the cases
spelling
remove instead of replace with ?
Fix test
Merge pull request #7 from skimhub/feature/illegalunicode

Feature/illegalunicode